1. Field of the Invention
The present invention relates to a coil chip used at high frequencies for use in a small size and light weight electronic devices such as cellular phones or personal digital assistants (PDAs). More specifically, the present invention relates to a helical coil chip having high Q characteristics that is compact, short and lightweight enough to be equipped in various modules in cellular phones. The present invention also relates to a method for producing such a helical coil chip.
2. Related Background Art
In recent years, downsizing and weight reduction of mobile communication devices such as cellular phones have been drastically achieved. Consequently, downsizing, length reduction and weight reduction of high frequency coil chips to be equipped in chips and various modules used in those devices have also been required. So far, the size of the coil chip has been reduced to 1 mm or less in coil chip length and 0.5 mm or less in coil diameter (or width).
Such coil chips have been conventionally produced by winding a wire directly on a bobbin like in the case of producing larger coils, as disclosed for example in Japanese Patent Application laid-Open No. 2000-252127. However, it is considered impossible under the present circumstances to realize further downsizing with that production process, and therefore new producing technology is desired. At present, technologies using a non-winding process with which further downsizing of coil chips can be realized have been conceived and developed for practical application. Such technologies include, for example, a laser cutting process disclosed in Japanese Patent Application Laid-Open No. H11-204362 or a thin film formation technology disclosed in Japanese Patent Application Laid-Open No. H11-283834.
In the laser cutting process, a material to be formed into wound wire is applied as a coating film that covers a core member, and then the coating film is processed into a thin wire(s) using a laser beam. However, this process involves the disadvantage that the material for the core member may be restricted in view of the effects of laser irradiation. In addition, the processed surface may suffer from surface roughness after cutting by a laser beam, and therefore there is the risk that wire intervals can become irregular due to the surface roughness if the wire intervals are to be further decreased. In view of the above, it is considered that this process suffers from many problems to be solved when more compact coil chips are to be produced in the future.
In a production method using thin film forming technology, which is considered to be the most developed practical technology, several layers of coil patterns are connected through via-holes formed on insulating layers. However, in that method, when the coil chip is made more compact and the wire formed thereon is made thinner, it would be difficult to stop up the via-holes that have a significant length and a minute diameter corresponding to the wire. In addition, since it is practically impossible in that method to arrange a wound wire on the outermost surface, the method is structurally unfavorable for use in producing coils having high Q characteristics.
Generally, when coil chips having different cross sectional areas and the same inductance are to be produced, the larger the cross sectional area of the coil is, the smaller the number of windings of the coil should be. Therefore, if a coil is formed on the outermost surface of a chip, a larger inductance can be obtained even when the size and the number of windings of the chip is the same. When the cross sectional area of a coil is made small, it is necessary to increase the number of windings of the coil in order to maintain the inductance. However, an increase in the number of windings of the coil causes an increase in the direct current resistance of the coil and an increase in leak current between wound wires to lead to a decrease in the Q value.
Furthermore, an increase in the number of windings of a coil exaggerates the influence of dielectric loss caused by the dielectric material used for the core member of the coil. The dielectric loss increases with an increase in the frequency of the signal applied to the coil chip. As described above, it is difficult for the aforementioned thin film forming process to produce a coil chip having a wound wire formed on the outermost surface thereon, and therefore that process is considered to be unsuitable for use in producing coil chips for higher frequency applications.
Furthermore, when a coil chip is made compact, the capacitance between terminal electrodes for example is no more negligible when applications for ultra high frequencies are brought into view. In this case, in order to obtain a high Q, it is necessary to do away with opposed electrodes to reduce the capacitance between the electrodes and to make the resonance frequency related to the inductance of the coil and the capacitance between the electrodes higher than the used signal frequency. The larger the inductance is and the higher the used frequency is, the more greatly the influence of the capacitance between the electrodes is exaggerated. It has been difficult in normal chip coils in which terminal electrodes are opposed to each other to reduce capacitance.